I recommend putting the username and password in notepad so they're right in front of you. Then copy and paste from there to make sure you can log into SFDC yourself using them.
Then copy and paste them into Admin > Salesforce, and paste in the new security token (this should have been sent by SFDC when you changed the password). You can also manually reset your security token by clicking your name in the top-right corner, and selecting My Settings from the dropdown, and then choosing Personal > Reset My Security Token from the left side.